Power converter and image processing apparatus

ABSTRACT

A power converter includes an insulating transformer including primary and secondary windings, a switching circuit configured to switch voltages applied to the primary winding, a power supply circuit connected to the secondary winding and configured to rectify and smooth a current flowing in the secondary winding and generate a DC voltage, an adjustment circuit configured to adjust the voltage and output the adjusted voltage that is a first voltage in a first mode and a second voltage that is less than the first voltage in a second mode, and a protection circuit including a first Zener diode and a second Zener diode and configured to stop output of the adjusted voltage. A cathode of the first diode is electrically connected to the power supply circuit. The second diode is connected in series to the first diode in the first mode and being short-circuited in the second mode.

FIELD

Embodiments described herein relate generally to a power converter andan image processing apparatus.

BACKGROUND

Power converters for converting electric power are widely used. Thepower converter supplies power via a transformer. The power converterhas a switching control circuit, typically an integrated circuit (IC),that controls a switching element to allow an alternating voltage toflow to a first side of the transformer. The switching control circuitoperates using a voltage generated in an auxiliary winding, which iscoupled to the transformer. The power converter includes a shuntregulator on a second side. The shunt regulator adjusts the voltageoutput from the second side of the transformer to a load. Theadjustments by the shunt regulator are based on a reference voltageapplied to a reference voltage terminal. The power converter adjusts theoutput voltage of the transformer to be a predetermined voltage byoperation of the shunt regulator. The power converter also includes anovervoltage protection circuit for stopping the switching controlcircuit when the output voltage of the transformer equals or exceedssome threshold voltage level.

The power converter can switch voltage division ratios in the shuntregulator by controlling the connection of a switch or jumper to aresistor connected to the reference voltage terminal of the shuntregulator. Thus, the power converter can output desired voltages. Also,the threshold voltage level for the overvoltage protection circuit canbe changed by controlling the connection of the switch or jumper.However, when the output voltage and the threshold voltage can be setindependently, the protection function might not be operated normally orthe protection function might be excessively operated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an image processing apparatus according to anembodiment.

FIG. 2 is a diagram of a power supply unit according to an embodiment.

FIGS. 3 and 4 are circuit diagrams of a power conversion circuitaccording to an embodiment.

DETAILED DESCRIPTION

In one embodiment, a power converter comprises an insulating transformerincluding a primary winding and a secondary winding, a switching circuitconfigured to switch voltages applied to the primary winding, a powersupply circuit connected to the secondary winding and configured torectify and smooth a current flowing in the secondary winding andgenerate a DC voltage, an adjustment circuit configured to adjust thegenerated DC voltage and output the adjusted DC voltage that is a firstDC voltage in a first mode and a second DC voltage that is less than thefirst DC voltage in a second mode, and a protection circuit including afirst Zener diode and a second Zener diode and configured to stop outputof the adjusted DC voltage. A cathode of the first Zener diode iselectrically connected to the power supply circuit, and the second Zenerdiode is connected in series to the first Zener diode in the first modeand being short-circuited in the second mode.

Hereinafter, embodiments will be described with reference to theaccompanying drawings.

FIG. 1 is a diagram of an image processing apparatus 1 according to oneembodiment.

The image processing apparatus 1 is a multi-function printer (MFP) thatperforms various processes such as image formation on a recording mediumsuch as a printing medium. For example, the image processing apparatus 1is a solid-state scanning type printer (for example, an LED printer)having an LED array. In some examples, the image processing apparatus 1may be an ink jet printer having an ink jet head for ejecting ink, ormay be another type of printer.

In the present embodiment, the image processing apparatus 1 charges aphotosensitive drum and then irradiates the photosensitive drum withlight corresponding to the image data for printing, thereby forming anelectrostatic latent image on the photosensitive drum. The imageprocessing apparatus 1 attaches toner to the electrostatic latent imageformed on the photosensitive drum, transfers the toner adhering to thelatent image on to a printing medium (such a sheet of paper or thelike), and thus forms a toner image on the printing medium. The imageprocessing apparatus 1 next applies heat and pressure to the printingmedium to which the toner image has been transferred, thereby fixing thetoner image on the printing medium.

The image processing apparatus 1 includes a system controller 11, acommunication interface 12, a display 13, an operation interface 14, animage reader 15, a conveying unit 16, an image forming unit 17, and apower supply unit 18. The system controller 11, the communicationinterface 12, the display 13, the operation interface 14, the imagereader 15, the conveying unit 16, the image forming unit 17, and thepower supply unit 18 are installed in a housing.

The system controller 11 controls the image processing apparatus 1. Thesystem controller 11 includes, for example, a processor 21 and a memory22.

The processor 21 is an arithmetic element (for example, a CPU (CentralProcessing Unit)) for executing arithmetic processing. The processor 21performs various processes on the basis of data such as programs storedin the memory 22. The processor 21 functions as a control unit capableof executing various operations by executing a program stored in thememory 22.

The memory 22 is a storage device or the like that can be used forstoring programs and data used in the programs. The memory 22 caninclude a volatile memory that temporarily stores data or the like beingprocessed by the processor 21. The memory 22 may further include anonvolatile memory for storing the programs.

The communication interface 12 is an interface for communicating with aclient device or the like which issues a print job via a network.

The display 13 displays a screen or screens in accordance with a signalsupplied from a display control unit, such as the system controller 11or a graphics controller. For example, the display 13 displays screensrelated to various settings of the image processing apparatus 1.

The operation interface 14 can have or include various input devices.The operation interface 14 supplies an operation signal corresponding tothe operation made via the input device to the system controller 11. Theinput devices can be, but are not limited to, a touch sensor, a numerickeypad, a power-on key, a paper feed key, various function keys, and akeyboard. The touch sensor may be, for example, a resistive touchsensor, a capacitive touch sensor, or the like. The touch sensoracquires information indicating a position specified by a user in acertain region of the touch sensor. The touch sensor can be configuredas a touch panel integrated with the display 13, so that the signal fromthe touch sensor indicates a touched position on a screen displayed onthe display 13. Thus, a user touch selection can be input to the systemcontroller 11.

The image reader 15 is configured to read an image printed on adocument. The image reader 15 includes, for example, a scanner and anautomatic document feeder (ADF). The scanner moves along a documentwhile the document is on a glass plate (document platen) to acquire animage of the entire document. The image reader 15 can similarly acquirean image of the entire document by feeding/moving the document past astationary scanner using the ADF.

The conveying unit 16 conveys a printing medium to the image formingunit 17, and then discharges the printing medium on which an image hasbeen formed by the image forming unit 17 from the housing. The conveyingunit 16 conveys printing media stored in a sheet cassette (not shown)one by one to the image forming unit 17. Further, the conveying unit 16discharges the printing after the image has been formed by the imageforming unit 17 to a paper discharge tray outside the housing.

The image forming unit 17 forms an image on a printing medium based onthe control of the system controller 11. The image forming unit 17includes at least a process unit, an exposure device, a transfermechanism, and a fixing device. In some examples, the image forming unit17 includes a process unit and an exposure device for each color oftoner provided in the image forming unit. For example, the colors oftoner can be cyan (C), magenta (M), yellow (Y), and black (K). The imageforming unit 17 can fix a toner image of each color (CMYK) to theprinting medium to form a color image.

The process unit in this example includes a photosensitive drum, acharger, and a development device. The photosensitive drum is acylindrical drum with a photosensitive layer formed on an outerperipheral surface of the drum. In general, the photosensitive drum isrotated at a constant speed by a driving mechanism.

The charger uniformly charges the surface of the photosensitive drum.For example, the charger electrostatically charges the photosensitivedrum to a particular negative potential by applying a voltage to thephotosensitive drum using a charging roller.

The development device is an apparatus for attaching toner to thephotosensitive drum. The development device includes a developer storagecontainer, a stirring mechanism, a developing roller, a doctor blade,and the like.

The developer container is a container for storing toner discharged froma toner cartridge. A toner carrier is generally stored in the developercontainer in advance. The toner from the toner cartridge is stirred andmixed with the carrier by the stirring mechanism to form a developer.The carrier is typically stored in the developer container in themanufacturing process of the development device.

The developing roller rotates within the developer container to adherethe developer to the surface of the developing roller. The doctor bladeis a member disposed at a predetermined distance from the surface of thedeveloping roller. The doctor blade removes any excess portions of thedeveloper adhering to the surface of the rotating developing roller. Asa result, a layer of the developer having a thickness corresponding tothe distance between the doctor blade and the surface of the developingroller is formed on the surface of the developing roller.

In this example, the exposure device includes a plurality of lightemitting elements. The exposure device is used to form a latent image onthe photosensitive drum by selectively irradiating the photosensitivedrum with light from the light emitting elements. The light emittingelements are, for example, light-emitting diodes (LED) or the like. Eachof the light emitting elements is configured to irradiate one point onthe photosensitive drum. The plurality of light emitting elements arearranged in a row along a main scanning direction, which is a directionparallel to the rotation axis of the photosensitive drum.

The exposure device irradiates the photosensitive drum with the lightemitting elements arranged along the main scanning direction, therebyforming one line of a latent image on the photosensitive drum. Theexposure device forms the entire latent image (which comprises aplurality of different lines) by irradiating the rotating photosensitivedrum.

In the above structure, when light is emitted from the exposure deviceonto the surface of the photosensitive drum that has been charged by thecharger, an electrostatic latent image is formed. When the layer ofdeveloper formed on the surface of the developing roller is broughtclose to the surface of the photosensitive drum, toner contained in thedeveloper adheres to the still charged portions electrostatic latentimage formed on the surface of the photosensitive drum. As a result, atoner image is formed on the surface of the photosensitive drumcorresponding to the electrostatic latent image.

The transfer mechanism transfers the toner image formed on the surfaceof the photosensitive drum to a printing medium. In this example, thetransfer mechanism includes a first transfer belt, a secondary transferroller, and a plurality of other rollers. The transfer mechanismreceives the toner image from the surface of the photosensitive drum onan outer peripheral surface of the first transfer belt. The transfermechanism conveys the toner image on the first transfer belt to atransfer nip at which the secondary transfer roller and the firsttransfer belt are in close contact with each other. The transfermechanism thus transfers the toner image from the first transfer belt tothe printing medium by passing a printing medium supplied by theconveying unit 16 through the transfer nip.

The fixing device fixes the toner image to the printing medium. Thefixing device includes a heat roller for applying heat to a printingmedium, a pressing roller for applying pressure to the printing medium,and a heater for heating the heat roller. The pressing roller forms afixing nip with the heat roller at which the pressing roller and theheating roller are brought into close contact with each other and thepressing roller applies pressure to the heat roller. The fixing deviceapplies heat and pressure to the printing medium passing the printingmedium through the fixing nip. Thereby, the fixing device fixes thetoner image formed on the printing medium. The printing medium which haspassed through the fixing nip is then discharged to the paper dischargetray by the conveying unit 16.

The power supply unit 18 is configured to supply power to the systemcontroller 11 of the image processing apparatus 1, the communicationinterface 12, the display 13, the operation interface 14, the imagereader 15, the conveying unit 16, and the image forming unit 17. FIG. 2is an explanatory diagram of the power supply unit 18. As shown in FIG.2, the power supply unit 18 includes a filter circuit 31, a rectifiercircuit 32, and a power conversion circuit 33.

The filter circuit 31 is connected between the commercial power supplyAC and the rectifier circuit 32. The filter circuit 31 is a noiseremoval circuit that prevents noise in a power conversion circuit 33from leaking to the commercial power supply AC. The filter circuit 31is, for example, an EMC (Electromagnetic Compatibility) filter. Thefilter circuit 31 includes, for example, a capacitor.

The rectifier circuit 32 rectifies the AC voltage received from thecommercial power supply AC via the filter circuit 31, and supplies apulsating positive voltage to a circuit in a subsequent stage. That is,the rectifier circuit converts the AC voltage into a positive voltageand supplies this positive voltage to the power conversion circuit 33.The rectifier circuit 32 is a full-wave rectifier circuit including aplurality of diodes, for example.

The power conversion circuit 33 is a circuit that supplies a DC voltagehaving a particular voltage value to the load. The power conversioncircuit 33 has a first side to which electric power is supplied and asecond side from which electric power is output. Depending on the statesof an output selecting switch S1, the power conversion circuit 33switches the DC voltages to be a first voltage (24V) or a second voltage(12V). A state in which the first voltage (24V) is output will bereferred to as a first mode, and a state in which the second voltage(12V) is output will be referred to as a second mode.

FIG. 3 is an explanatory circuit diagram of the power conversion circuit33. First, the configuration on the first side of the power conversioncircuit 33 will be described. The power conversion circuit 33 includes aDC power source 41, an insulating transformer 42, and a switch circuit43 on the first side. Furthermore, a power factor improvement circuitmay be provided on the first side of the power conversion circuit 33.

The DC power source 41 is a circuit for supplying a DC voltage to theinsulating transformer 42. The DC power source 41 includes a smoothingcapacitor C1. The smoothing capacitor C1 stores electric charges by thepulsating positive voltage supplied from the rectifier circuit 32 andsupplies the smoothed DC voltage to the circuits connected in parallel.

The insulating transformer 42 includes a primary winding L1 whichgenerates a magnetic field, and a secondary winding L2 which is isolatedfrom the primary winding L1 and excited by the magnetic field generatedby the primary winding L1. The insulating transformer 42 furtherincludes an auxiliary winding L3 which is excited by a magnetic fieldgenerated by the primary winding L1 to supply electric power to theswitch circuit 43. That is, the secondary winding L2 and the auxiliarywinding L3 are electromagnetically coupled to the primary winding L1.

The switch circuit 43 is a circuit for switching on and off the currentflowing from the smoothing capacitor C1 to the primary winding L1. Theswitch circuit 43 includes a semiconductor switch S2, a control circuit44, a diode D1, a smoothing capacitor C2, a photocoupler PC1, and aphotocoupler PC2.

The semiconductor switch S2 is a semiconductor switch for switching theconduction state based on control of the control circuit 44. Based onthe control of the control circuit 44, the semiconductor switch S2 turnson and off the current flowing from the smoothing capacitor C1 to theprimary winding L1. The semiconductor switch S2 is, for example, ann-type channel FET. A drain terminal of the semiconductor switch S2 isconnected to the primary winding L1, a source terminal of thesemiconductor switch S2 is connected to a low potential side of thesmoothing capacitor C1, and a gate terminal of the semiconductor switchS2 is connected to the control circuit 44. The semiconductor switch S2switches between the conduction state in which the drain terminal andthe source terminal are conducted (i.e., ON state), and thenon-conduction state in which the drain terminal and the source terminalare non-conductive (i.e., OFF state), based on a control signal suppliedfrom the control circuit 44 to the gate terminal.

The control circuit 44 is a control circuit for controlling thesemiconductor switch S2. The control circuit 44 includes a terminalconnected to a high potential side of the smoothing capacitor C2, aterminal connected to the gate terminal of the semiconductor switch S2,a terminal connected to the photocoupler PC1 (hereinafter called theoutput control terminal, and a terminal connected to the photocouplerPC2 (hereinafter called overvoltage protection terminal). The controlcircuit 44 inputs a high-frequency pulse signal to the semiconductorswitch S2 as the control signal. The control circuit 44 inputs a pulsesignal to, for example, the gate terminal of the semiconductor switchS2. Thus, the control circuit 44 switches between the ON state and theOFF state of the semiconductor switch S2 at a high speed. As a result, ahigh frequency pulse is supplied to the primary winding L1 of theinsulating transformer 42 by the potential of the smoothing capacitorC1, and a magnetic field is generated by the primary winding L1.

As described above, the switch circuit 43 functions as a flybackconverter for converting a DC voltage into a high frequency pulse. Theswitch circuit 43 may be configured as another converter such as a halfbridge converter for supplying a high frequency pulse to the primarywinding L1 of insulating transformer 42 depending on the potential ofthe smoothing capacitor C1, or a full bridge converter.

The diode D1 rectifies the current generated in the auxiliary windingL3. The smoothing capacitor C2 is connected in parallel with theauxiliary winding L3 as viewed from the control circuit 44. Thesmoothing capacitor C2 stores electric charges by a current generated inthe auxiliary winding L3, and supplies a smoothed DC voltage to thecontrol circuit 44.

The photocoupler PC1 includes a light emitting diode LED1 and a phototransistor PT1. In the photocoupler PC1, when a current flows from theanode of the light-emitting diode LED1 to the cathode and thelight-emitting diode LED1 emits light, light interacts with thephototransistor PT1 such that the collector terminal and the emitterterminal of the phototransistor PT1 are electrically connected to eachother.

The light emitting diode LED1 of the photocoupler PC1 is provided on thesecond side of the power conversion circuit 33, and the photo transistorPT1 of the photocoupler PC1 is disposed on the first side of the powerconversion circuit 33. The collector terminal of the phototransistor PT1is connected to the output control terminal of the control circuit 44.The emitter terminal of the phototransistor PT1 is connected to the lowpotential side of the smoothing capacitor C1. The voltage between thecollector terminal and the emitter terminal of the phototransistor PT1changes in response to the light emitted from the light emitting diodeLED1.

The photocoupler PC2 includes a light emitting diode LED2 and a phototransistor PT2. In the photocoupler PC2, when a current flows from theanode of the light-emitting diode LED2 to the cathode and thelight-emitting diode LED2 emits light, the light interacts with thephototransistor PT2 such that the collector terminal and the emitterterminal of the phototransistor PT2 are electrically connected to eachother.

The light emitting diode LED2 of the photocoupler PC2 is provided on thesecond side of the power conversion circuit 33, and the photo transistorPT2 of the photocoupler PC2 is disposed on the first side of the powerconversion circuit 33. The collector terminal of the phototransistor PT2is connected to the overvoltage protection terminal of the controlcircuit 44. The emitter terminal of the phototransistor PT2 is connectedto the low potential side of the smoothing capacitor C1. That is, thephotocoupler PC2 connects the overvoltage protection terminal of thecontrol circuit 44 to the low potential side of the smoothing capacitorC1 in response to the emission of the light emitting diode LED2.

Next, the configuration of the second side of the power conversioncircuit 33 will be described.

The power conversion circuit 33 includes the secondary winding L2 of theinsulating transformer 42, a rectifying and smoothing circuit 45, avoltage adjustment circuit 46, and an overvoltage protection circuit 47of an insulating transformer 42 on the second side. A load in the imageprocessing apparatus 1 (e.g., the conveying unit 16, the image formingunit 17, the system controller 11, and the like) is connected to anoutput terminal on the second side of the power conversion circuit 33.

The secondary winding L2 is excited in accordance with the magneticfield generated by the primary winding L1, thereby generating electricpower. A voltage corresponding to the ratio of the number of turns ofthe primary winding L1 and the secondary winding L2 is generated in thesecondary winding L2.

The rectifying and smoothing circuit 45 is a circuit for rectifying andsmoothing electric power generated in the secondary winding L2. Therectifying and smoothing circuit 45 includes a diode D2 and a smoothingcapacitor C3.

The diode D2 has an anode connected to the secondary winding L2, and acathode connected to the high potential side of the smoothing capacitorC3. The diode D2 rectifies the current generated in the secondarywinding L2, and supplies the rectified electric current to the smoothingcapacitor C3. In some examples, the diode D2 may be replaced by a MOSFETand a synchronous rectification control IC. In such a case, thesynchronous rectification can be performed by the MOSFET.

The smoothing capacitor C3 smooths the positive voltage supplied fromthe diode D2. The smoothing capacitor C3 supplies a DC voltage to thecircuits connected in parallel. The high potential side of the smoothingcapacitor C3 is connected to the output terminal of the power conversioncircuit 33. That is, the load is connected to the smoothing capacitorC3. The smoothing capacitor C3 supplies a smoothed DC voltage to theload (for example, the image forming unit 17). That is, the rectifyingand smoothing circuit 45 functions as a DC power source which outputs aDC voltage.

The voltage adjustment circuit 46 adjusts the voltage at the outputterminal of the power conversion circuit 33. The voltage adjustmentcircuit 46 draws a current from the smoothing capacitor C3, and adjuststhe voltage at the output terminal of the power conversion circuit 33 toa predetermined voltage in accordance with the amount of current.

The voltage adjustment circuit 46 includes a shunt regulator 51, aresistor R1, a resistor R2, a resistor R3, a resistor R4, a resistor R5,a resistor R6, a diode D3, the light emitting diode LED1 of thephotocoupler PC1, a switch Q1, and an output selecting switch S1. Thevoltage adjustment circuit 46 can pull down or up the potential of thehigh potential side of the smoothing capacitor C3 in accordance with thevoltage dividing ratio of the shunt regulator 51.

The shunt regulator 51 is a constant voltage regulator for adjusting thevoltage at the output terminal of the power conversion circuit 33. Theshunt regulator 51 includes an input terminal, an output terminal, and areference voltage terminal. The shunt regulator 51 includes a transistorprovided in parallel with the load (that is, between the high potentialside of the smoothing capacitor C3 and the GND) and an error amplifier.In the shunt regulator 51, the error amplifier controls the transistorbased on the potential of the reference voltage terminal, therebycausing a current to flow from the input terminal to the outputterminal.

The input terminal of the shunt regulator 51 is a collector terminal ofthe transistor. The input terminal of the shunt regulator 51 isconnected to the high potential side of the smoothing capacitor C3 viathe light emitting diode LED1 of the photocoupler PC1 and the resistorR1 connected in series.

The output terminal of the shunt regulator 51 is an emitter terminal ofthe transistor. The output terminal of the shunt regulator 51 isconnected to GND.

The reference voltage terminal of the shunt regulator 51 is an inputterminal of the error amplifier. The output terminal of the erroramplifier is connected to the base terminal of the transistor. Thereference voltage terminal of the shunt regulator 51 is connected to thehigh potential side of the smoothing capacitor C3 via the resistor R2.The reference voltage terminal of the shunt regulator 51 is connected toGND via the resistor R3. Further, the reference voltage terminal of theshunt regulator 51 is connected to an output terminal of a switch Q1(i.e., an emitter terminal), which will be described later, via theresistor R4.

The switch Q1 is a semiconductor switch, and is, for example, an NPNtransistor. The switch Q1 may be an n-type MOSFET.

The base terminal of the switch Q1 is connected to a connection pointbetween the resistor R1 and the resistor R6. The emitter terminal of theswitch Q1 is connected to the reference voltage terminal of the shuntregulator 51 via the resistor R4. The collector terminal of the switchQ3 is connected to the high potential side of the smoothing capacitorC1.

The output selecting switch S1 may be, for example, a dip switch, atoggle switch, a rocker switch, or a thumb rotary switch. Further, theoutput selecting switch S1 may be a jumper to change terminals to beconnected. The output selecting switch S1 is a switch for switchingbetween a conduction state (ON) and a non-conduction state (OFF) by ahuman operation. The output selecting switch S1 is connected between thecathode of the diode D3 and GND. For example, the output selectingswitch S1 is provided outside of the power supply unit 18 so that anoperator can easily switch the states1.

The overvoltage protection circuit 47 is a circuit for preventing anovervoltage (i.e., a voltage greater than or equal to a rated voltage)from being output from the power conversion circuit 33. The overvoltageprotection circuit 47 stops the output of the rectifying and smoothingcircuit 45 which is the DC power source when the output voltage becomesequal to or greater than the threshold voltage. For example, theovervoltage protection circuit 47 notifies the control circuit 44 of thefirst side that the overvoltage is being output, thereby stopping theswitching operation of the control circuit 44.

The overvoltage protection circuit 47 includes a switch Q2, a switch Q3,a switch Q4, a resistor R7, a resistor R8, a resistor R9, a first Zenerdiode ZD1, a diode D4, a second Zener diode ZD2, and the light emittingdiode LED2 of the photocoupler PC2.

The switch Q2 is a semiconductor switch, and is, for example, a PNPtransistor. The switch Q2 may be a p-type MOSFET.

The base terminal of the switch Q2 is connected to the cathode of thediode D3 and the anode of the diode D4 whose cathode is connected to theoutput selecting switch S1. The emitter terminal of the switch Q2 isconnected to the high potential side of the smoothing capacitor C3(i.e., the output terminal of the rectifying and smoothing circuit 45)via the resistor R7. The collector terminal of the switch Q2 isconnected to GND via the resistor R8.

The switch Q3 is a semiconductor switch, and is, for example, an NPNtransistor. The switch Q3 may be an n-type MOSFET.

The base terminal of the switch Q3 is connected to the collectorterminal of the switch Q2. The emitter terminal of the switch Q3 isconnected to GND. The collector terminal of the switch Q3 is connectedto the anode of the first Zener diode ZD1 and the cathode of the secondZener diode ZD2, which will be described later.

The switch Q4 is a semiconductor switch, and is, for example, a PNPtransistor. The switch Q4 may be a p-type MOSFET.

The base terminal of the switch Q4 is connected to the cathode of thefirst Zener diode ZD1. The emitter terminal of the switch Q4 isconnected to the high potential side of the smoothing capacitor C3(i.e., the output terminal of the rectifying and smoothing circuit 45).The collector terminal of the switch Q4 is connected to GND via theresistor R9 and the light emitting diode LED2 of the photocoupler PC2.

The anode of the first Zener diode ZD1 is connected to the collectorterminal of the switch Q3 and the cathode of the second Zener diode ZD2.The cathode of the first Zener diode ZD1 is connected to the baseterminal of the switch Q4.

The anode of the second Zener diode ZD2 is connected to GND. The cathodeof the second Zener diode ZD2 is connected to the anode of the firstZener diode ZD1 and the collector terminal of the switch Q3.

The anode of the light emitting diode LED2 of the photocoupler PC2 isconnected to the collector terminal of the switch Q4 via the resistorR9. The cathode of the light emitting diode LED2 of the photocoupler PC2is connected to GND. The light emitting diode LED2 of the photocouplerPC2 is provided at a position where a current flows in response to theconduction of the first Zener diode ZD1.

The operating configuration of the power conversion circuit 33 havingthe above-described configuration is changed depending on the states ofthe output selecting switch S1. First, the operation of the powerconversion circuit 33 in the case where the output selecting switch S1is OFF (i.e., the first mode in which the DC voltage of 24V is output)will be described.

(State of each Switch in Voltage Adjustment Circuit 46)

When the output selecting switch S1 is off, no current flows through theresistor R5, the resistor R6, and the diode D3 connected in series.Therefore, the switch Q1 is also turned off.

(State of each Switch of the Overvoltage Protection Circuit 47).

In the voltage adjustment circuit 46, when the output selecting switchS1 is OFF, the current does not flow through the base terminal of theswitch Q2 of the overvoltage protection circuit 47, and thus the switchQ2 is turned off.

Further, when the switch Q2 is OFF, no current flows through the baseterminal of the switch Q3, and thus the switch Q3 is also turned OFF.

(Operation of Shunt Regulator 51)

When the switch Q1 is OFF, the resistor R2 and the resistor R3 areconnected to the reference voltage terminal of the shunt regulator 51.In this case, the voltage Vout1 is output from the output terminal ofthe power conversion circuit 33, which is adjusted by the shuntregulator 51 based on the reference voltage Vref, which is the voltageapplied to the reference voltage terminal of the shunt regulator 51, andthe voltage division ratio between the resistor R2 and the resistor R3.The voltage Vout1 is calculated by the following equation:

Vout1=(1+R2/R3)×Vref.

Note that the voltage Vout1 is, for example, 24V.

Further, the photo transistor PT1 of the photocoupler PC1 is turned onby the current drawn into the shunt regulator 51, and the light emittingdiode LED1 of the photocoupler PC1 emits light. When the light emittingdiode LED1 of the photocoupler PC1 emits light, the photo transistor PT1of the photocoupler PC1 on the first side of the power conversioncircuit 33 is turned on. As a result, a current flows from the controlcircuit 44 to the low potential side of the smoothing capacitor C1 viathe phototransistor PT1 on the first side. The current value of thecurrent flowing through the phototransistor PT1 varies depending on thecurrent flowing through the light emitting diode LED1 of thephotocoupler PC1, i.e., the current drawn into the shunt regulator 51.

(Operation of the Overvoltage Protection Circuit 47)

When the switch Q2 and the switch Q3 are OFF, the first Zener diode ZD1and the second Zener diode ZD2 are connected in series between the baseterminal of the switch Q4 and GND. In this case, the potentialdifference between the potential of the emitter terminal of the switchQ4 (that is, the potential of the output terminal of the rectifying andsmoothing circuit 45=output voltage=Vout1) and GND is applied to thefirst Zener diode ZD1 and the second Zener diode ZD2. When the outputvoltage becomes equal to or higher than the sum of the Zener voltage ofthe first Zener diode ZD1 and the Zener voltage of the second Zenerdiode ZD2 (hereinafter called the first threshold voltage), the switchQ4 is turned on. The first threshold voltage is, for example, 27V. Whenthe switch Q4 is turned on, a current flows through the resistor R9 andthe light emitting diode LED 9 of the photocoupler PC2, and the lightemitting diode LED2 of the photocoupler PC2 emits light. When the lightemitting diode LED2 of the photocoupler PC2 emits light, the phototransistor PT2 of the photocoupler PC2 is turned on in the first side.As a result, a current flows from the overvoltage protection terminal ofthe control circuit 44 to the low potential side of the smoothingcapacitor C1 through the phototransistor PT2 on the first side of thepower conversion circuit 33.

(Operation of Control Circuit 44)

The control circuit 44 controls the semiconductor switch S2 to supply ahigh frequency pulse to the primary winding L1 of the insulatingtransformer 42 by the potential of the smoothing capacitor C1, therebygenerating a magnetic field in the primary winding L1.

The control circuit 44 controls the on/off duty ratio of the pulsesignal to be input to the semiconductor switch S2 based on the currentflowing from the output control terminal to the low potential side ofthe smoothing capacitor C1 via the phototransistor PT1 of thephotocoupler PC1. As described above, the voltage between the emitterterminal and the collector terminal of the phototransistor PT1 of thephotocoupler PC1 changes in accordance with the emission of the lightemitting diode LED1. That is, the voltage between the emitter terminaland the collector terminal of the phototransistor PT1 of thephotocoupler PC1 varies according to the current flowing from therectifying and smoothing circuit 45 on the second side of the powerconversion circuit 33 to the input terminal of the shunt regulator 51.The control circuit 44 controls the on/off duty ratio of the pulsesignal to be inputted to the semiconductor switch S2 based on thecurrent flowing to the low potential side of the smoothing capacitor C1or the voltage between the collector terminal and the emitter terminalof the phototransistor PT1 via the phototransistor PT1 of thephotocoupler PC1. Thus, the control circuit 44 controls the intensity ofthe magnetic field generated in the primary winding L1 of the insulatingtransformer 42. Thus, the electric power supplied to the second side iscontrolled.

In addition, when a current flows from the overvoltage protectionterminal to the low potential side of the smoothing capacitor C1 throughthe phototransistor PT2, the control circuit 44 stops the operation ofthe semiconductor switch S2. That is, the control circuit 44 stops theoperation of the semiconductor switch S2 when the output voltage on thesecond side becomes equal to or greater than the first threshold voltagecorresponding to the sum of the Zener voltage of the first Zener diodeZD1 and the Zener voltage of the second Zener diode ZD2. Thus, the powerconversion circuit 33 stops the output of the rectifying and smoothingcircuit 45 which is the DC power source, thereby preventing the outputvoltage exceeding the rated value from continuing to be output.

Next, the operation of the power conversion circuit 33 in the case wherethe output selecting switch S1 is ON (i.e., the second mode in which theDC voltage of 12V is output) will be described.

(State of each Switch in Voltage Adjustment Circuit 46).

When the output selecting switch S1 is on, a current flows through theresistor R5, the resistor R6, and the diode D3 connected in series, andthe switch Q1 is turned on.

(State of each Switch of the Overvoltage Protection Circuit 47).

In the voltage adjustment circuit 46, when the output selecting switchS1 is turned on, a current flows from the base terminal of the switch Q2of the overvoltage protection circuit 47. As a result, the switch Q2 isturned on. When the switch Q2 is turned on, the output voltage of therectifying and smoothing circuit 45 is applied to the base terminal ofthe switch Q3 via the resistor R7 and the switch Q2. As a result, theswitch Q3 is turned on.

(Operation of Shunt Regulator 51)

When the switch Q1 is ON, the resistor R2 and the resistor R4 areconnected in parallel between the reference voltage terminal of theshunt regulator 51 and the high potential side of the smoothingcapacitor C3, and the resistor R3 is connected between the referencevoltage terminal of the shunt regulator 51 and GND. In this case, theresistor R2 and the resistor R4 conceptually make up a combined resistorR0, the resistance value of which can be calculated by the followingequation:

R0=R2×R4/(R2+R4).

Therefore, the voltage Vout2 output from the output terminal of thepower conversion circuit 33 can be calculated by the following equation:

Vout2=(1+R0/R3)×Vref.

Note that the combined resistance value R0 is smaller than theresistance value R2. Therefore, the voltage dividing ratio of the shuntregulator 51 is changed, and the voltage Vout2 is smaller than thevoltage output when the switch Q1 is off. The voltage Vout2 is, forexample, 12V.

(Operation of the Overvoltage Protection Circuit)

When the switch Q2 and the switch Q3 are turned on, the anode of thefirst Zener diode ZD1 is connected to GND via the switch Q3. That is, ashort circuit path is formed in parallel with the second Zener diodeZD2. In this case, a potential equal to the difference between thepotential of the emitter terminal of the switch Q4 (that is, thepotential of the output terminal of the rectifying and smoothing circuit45=output voltage=Vout2) and GND is applied to the first Zener diodeZD1. Since the short circuit path is formed in parallel with the secondZener diode ZD2, the output voltage is insubstantially applied to thesecond Zener diode ZD2. When the output voltage becomes equal to orhigher than the Zener voltage of the first Zener diode ZD1 (hereinaftercalled the second threshold voltage), the switch Q4 is turned on. Thesecond threshold voltage is, for example, 15V. When the switch Q4 isturned on, a current flows through the resistor R9 and light emittingdiode LED2 of the photocoupler PC2, and the light emitting diode LED2emits light. When the light emitting diode LED2 emits light, the phototransistor PT2 of the photocoupler PC2 on the first side of the powerconversion circuit 33 is turned on. As a result, a current flows fromthe overvoltage protection terminal of the control circuit 44 to the lowpotential side of the smoothing capacitor C1 through the phototransistorPT2 in the first side.

(Operation of Control Circuit)

When the output selecting switch S1 is on, the control circuit 44controls the semiconductor switch S2 to supply a high frequency pulse tothe primary winding L1 of the insulating transformer 42 by the potentialof the smoothing capacitor C1, thereby generating a magnetic field inthe primary winding L1.

The control circuit 44 stops the operation of the semiconductor switchS2 when a current flows from the overvoltage protection terminal to thelow potential side of the smoothing capacitor C1 via the phototransistorPT2. That is, in the control circuit 44, when the output voltage on thesecond side becomes equal to or greater than the second thresholdvoltage corresponding to the Zener voltage of the first Zener diode ZD1,the operation of the semiconductor switch S2 is stopped. Thus, the powerconversion circuit 33 stops the output of the rectifying and smoothingcircuit 45 which is the DC power source, thereby preventing the outputvoltage exceeding the rated value from being output.

As described above, the power conversion circuit 33 is configured tosupply electric power to from the first side to the second side in astate in which the first side and the second side are electricallyisolated from each other by the insulating transformer 42. The powerconversion circuit 33 includes the smoothing capacitor C1 functioning asthe DC power supply, the semiconductor switch S2, and the controlcircuit 44 on the first side. The insulating transformer 42 includes theprimary winding L1, the secondary winding L2 electromagnetically coupledto the primary winding L1, and the auxiliary winding L3. The controlcircuit 44 turns on and off a switch for switching the connectionbetween the smoothing capacitor C1 and the primary winding L1, therebygenerating a magnetic field in the primary winding L1.

Further, the power conversion circuit 33 includes the rectifiersmoothing circuit 45, the voltage adjustment circuit 46 including theshunt regulator 51, and the overvoltage protection circuit 47 on thesecondary side. The rectifying and smoothing circuit 45 is connected tothe secondary winding L2, rectifies and smooths the current generated inthe secondary winding L2, and outputs a DC voltage. The shunt regulator51 adjusts the output voltage of the rectifying and smoothing circuit 45based on the voltage of the reference voltage terminal connected to theoutput terminal of the rectifying and smoothing circuit 45 through theresistor R2 and grounded via the resistor R3. The overvoltage protectioncircuit 47 stops the output of the rectifying and smoothing circuit 45when the output voltage of the rectifying and smoothing circuit 45exceeds a threshold voltage.

The voltage adjustment circuit 46 switches the output voltages accordingto the states of the output selecting switch S1. For example, thevoltage adjustment circuit 46 operates in either the first mode in whichthe output selecting switch S1 is off and the output voltage from therectifying and smoothing circuit 45 is the first voltage (e.g., 24V), orthe second mode in which the output selecting switch S1 is on and theoutput voltage from the rectifying and smoothing circuit 45 is thesecond voltage (e.g., 12V). That is, the voltage adjustment circuit 46switches the first mode in which the output voltage of the rectifyingand smoothing circuit 45 which is the DC power source is set to thefirst voltage, and the second mode in which the output voltage of therectifying and smoothing circuit 45 is set to the second voltage whichis lower than the first voltage, according to the on/off states of theoutput selecting switch S1.

The overvoltage protection circuit 47 switches the threshold voltagesfor stopping the voltage output from the rectifying and smoothingcircuit 45 depending on the states of the output selecting switch S1.For example, in the first mode, the overvoltage protection circuit 47stops the voltage output from the rectifying and smoothing circuit 45when the voltage is equal to or greater than a first threshold (forexample, 27 V) corresponding to the sum of the Zener voltage of thefirst Zener diode ZD1 and the Zener voltage of the second Zener diodeZD2. Further, for example, in the second mode, the overvoltageprotection circuit 47 stops the voltage output from the rectifying andsmoothing circuit 45 when the voltage is equal to or greater than thesecond threshold voltage (for example, 15V or more) corresponding to theZener voltage of the first Zener diode ZD1. In the second mode, theovervoltage protection circuit 47 forms a short circuit path in parallelwith the second Zener diode ZD2, whereby the Zener voltage of the firstZener diode ZD1 is used as the second threshold voltage.

That is, in the overvoltage protection circuit 47, during the secondmode in which the output voltage of the rectifying and smoothing circuit45 is the first voltage (24V), the overvoltage protection thresholdvoltage is the first threshold voltage (for example, 27V), and in thefirst mode in which the output voltage of the rectifying and smoothingcircuit 45 is the second voltage (12V), the overvoltage protectionthreshold voltage is a second threshold voltage (for example, 15V). Inthis way, the overvoltage protection circuit 47 changes the overvoltageprotection thresholds in conjunction with the voltages output from therectifying and smoothing circuit 45. Thus, the power conversion circuit33 can prevent a state in which the protection function is not normallyoperated or a protection function is excessively operated if anerroneous threshold has been set.

The overvoltage protection circuit 47 is not limited to theabove-described configuration. In general, the overvoltage protectioncircuit 47 may have any configuration as long as the overvoltageprotection threshold voltage can be switched in conjunction with thefirst mode and the second mode.

FIG. 4 is an explanatory circuit diagram of an overvoltage protectioncircuit 47A which is a modification example of the overvoltageprotection circuit 47 described above.

The overvoltage protection circuit 47A includes a switch Q5, a switchQ6, a resistor R10, a resistor R11, a resistor R12, a diode D4, a firstZener diode ZD1, a second Zener diode ZD2, and a light emitting diodeLED2 of a photocoupler PC2.

The switch Q5 is a semiconductor switch, and is, for example, a PNPtransistor. The switch Q5 may be a p-type MOSFET.

The base terminal of the switch Q5 is connected to the cathode of thediode D3 via the diode D4. The emitter terminal of the switch Q5 isconnected to the high potential side of the smoothing capacitor C3(i.e., the output terminal of the rectifying and smoothing circuit 45)via the resistor R10. The collector terminal of the switch Q5 isconnected to GND via the resistor R11.

The switch Q6 is a semiconductor switch, and is, for example, a PNPtransistor. In some examples, the switch Q6 may be a p-type MOSFET.

The base terminal of the switch Q6 is connected to the emitter terminalof the switch Q5. The emitter terminal of the switch Q6 is connected tothe high potential side of the smoothing capacitor C3 (i.e., the outputterminal of the rectifying and smoothing circuit 45). The collectorterminal of the switch Q6 is connected to the cathode of the first Zenerdiode ZD1 and the anode of the second Zener diode ZD2.

The anode of the first Zener diode ZD1 is connected to the anode of thelight emitting diode LED2 of the photocoupler PC2 through the resistorR12. The cathode of the first Zener diode ZD1 is connected to the anodeof the second Zener diode ZD2 and to the collector terminal of theswitch Q6.

The anode of the second Zener diode ZD2 is connected to the cathode ofthe first Zener diode ZD2 and to the collector terminal of the switchQ6. The cathode of the second Zener diode ZD2 is connected to theemitter terminal of the switch Q6 and to the high potential side of thesmoothing capacitor C3 (i.e., the output terminal of the rectifying andsmoothing circuit 45).

The anode of the light emitting diode LED2 of the photocoupler PC2 isconnected to the anode of the first Zener diode ZD1 via the resistorR12. The cathode of the light emitting diode LED2 of the photocouplerPC2 is connected to GND.

The operation of the overvoltage protection circuit 47A in a case wherethe output selecting switch S1 is OFF will be described.

In the voltage adjustment circuit 46, when the output selecting switchS1 is off, no current flows through the base terminal of the switch Q5of the overvoltage protection circuit 47, and the switch Q5 is turnedoff. Further, since no current flows through the emitter terminal of theswitch Q5, no current flows to the base terminal of the switch Q6, andthe switch Q6 is also turned off.

When the switch Q5 and the switch Q6 are off, the first Zener diode ZD1and the second Zener diode ZD2 are connected in series between the highpotential side of the smoothing capacitor C3 (i.e., the output terminalof the rectifying and smoothing circuit 45) and GND. In this case, apotential difference between the high potential side of the smoothingcapacitor C3 (i.e., the output terminal of the rectifying and smoothingcircuit 45) and GND is applied to the first Zener diode ZD1 and thesecond Zener diode ZD2 via the resistor R12 and the light emitting diodeLED2 of the photocoupler PC2. When the output voltage becomes equal toor greater than the sum of the Zener voltage of the first Zener diodeZD1 and the Zener voltage of the second Zener diode ZD2 (i.e., the firstthreshold voltage), a current flows through the resistor R12 and thelight emitting diode LED2, and the light emitting diode LED2 emitslight. When the light emitting diode LED2 emits light, the phototransistor PT2 of the photocoupler PC2 on the first side of the powerconversion circuit 33 is turned on. The first threshold voltage is, forexample, 27V.

In the voltage adjustment circuit 46, when the output selecting switchS1 is turned on, a current flows from the base terminal of the switch Q5of the overvoltage protection circuit 47. As a result, the switch Q5 isturned on. When the switch Q5 is turned on, the output voltage of therectifying and smoothing circuit 45 is applied to the base terminal ofthe switch Q6 via the resistor R10 and the switch Q5. As a result, theswitch Q6 is turned on.

When the switch Q5 and the switch Q6 are on, the cathode of the firstZener diode ZD1 is connected to the output terminal of the rectifyingand smoothing circuit 45. That is, a short circuit path is formed inparallel with the second Zener diode ZD2. When the short circuit path isformed in parallel with the second Zener diode ZD2, the output voltageof the rectifying and smoothing circuit 45 is insubstantially applied tothe second Zener diode ZD2. Therefore, when the output voltage of therectifying and smoothing circuit 45 becomes equal to or higher than theZener voltage of the first Zener diode ZD1 (i.e., the second thresholdvoltage), the first Zener diode ZD1 is turned on, and a current flows.When a current flows through the first Zener diode ZD1, a current flowsalso through the resistor R9 connected in series to the first Zenerdiode ZD1 and the light emitting diode LED2 of the photocoupler PC2, andthe light-emitting diode LED2 of the photocoupler PC2 emits light. Whenthe light emitting diode LED2 of the photocoupler PC2 emits light, thephoto transistor PT2 of the photocoupler PC2 on the first side of thepower conversion circuit 33 is turned on. As a result, a current flowsfrom the overvoltage protection terminal of the control circuit 44 tothe low potential side of the smoothing capacitor C1 through thephototransistor PT2 on the first side of the power conversion circuit33. The second threshold voltage is, for example, 15V.

The control circuit 44 stops the operation of the semiconductor switchS2 when a current flows from the overvoltage protection terminal to thelow potential side of the smoothing capacitor C1 via the phototransistorPT2. That is, in the control circuit 44, when the voltage output fromthe second side becomes equal to or greater than the second thresholdvoltage corresponding to the Zener voltage of the first Zener diode ZD1,the operation of the semiconductor switch S2 is stopped. Thus, the powerconversion circuit 33 stops the output of the rectifying and smoothingcircuit 45 which is a DC power source, thereby preventing the outputvoltage exceeding the rated value from continuing to be output.

Also with the configuration described above, the power conversioncircuit 33 can prevent a state in which the protection function is notnormally operated or a protection function is excessively operated whenan erroneous threshold voltage is set.

Although the power conversion circuit 33 is incorporated in an imageprocessing apparatus 1 in the above embodiment, the present disclosureis not limited to this configuration. The power conversion circuit 33may be incorporated in any device, such as a commodity registrationdevice.

Although a Zener diode has been described as an overvoltage detectingelement in the above example embodiment, a comparator, a voltagemonitoring IC, or the like may instead be used to detect an overvoltageat an input terminal, and another element and an arrangement having afunction of generating a current and a voltage at the output terminalmay be used.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1. A power converter, comprising: a transformer including a firstwinding and a second winding; a switching circuit configured to switchvoltages applied to the first winding; a power supply circuit connectedto the second winding and configured to rectify and smooth a currentflowing in the second winding and generate a DC voltage; an adjustmentcircuit configured to adjust the generated DC voltage according to anoperation mode of the power converter including a first mode in whichthe generated DC voltage is adjusted to a first DC voltage and a secondmode in which the generated DC voltage is adjusted to a second DCvoltage that is less than the first DC voltage, and output the adjustedDC voltage; and a protection circuit including a first Zener diode and asecond Zener diode and configured to stop output of the adjusted DCvoltage, each of the first and second Zener diodes having a cathode andan anode, the cathode of the first Zener diode being electricallyconnected to the power supply circuit, the second Zener diode beingconnected in series to the first Zener diode in the first mode and beingshort-circuited in the second mode, the anode of one of the first andsecond Zener diodes being directly connected to the cathode of the otherof the first and second Zener diodes via a wire.
 2. The power converteraccording to claim 1, wherein the protection circuit further includes alight emitting diode (LED) that emits light when a current flows in thefirst Zener diode, and the switching circuit includes a phototransistorand is configured to stop applying the voltages to the first windingwhen a current flows in the phototransistor.
 3. The power converteraccording to claim 1, further comprising: a photocoupler comprising: alight emitting diode that emits light when a current flows in the firstZener diode, and a phototransistor connected to the switching circuitand coupled to the light emitting diode.
 4. The power converteraccording to claim 1, wherein the protection circuit further includes aswitch connected in parallel to the second Zener diode and is turned onin the second mode.
 5. The power converter according to claim 4, whereinthe anode of the first Zener diode is connected to both the cathode ofthe second Zener diode and the switch.
 6. The power converter accordingto claim 5, wherein the switch is a transistor including a collectorterminal connected to the cathode of the second Zener diode and anemitter terminal connected to the anode of the second Zener diode. 7.The power converter according to claim 4, wherein the switch isconnected in series to the first Zener diode and the power supplycircuit therebetween.
 8. The power converter according to claim 7,wherein the switch is a transistor including an emitter terminalconnected to the cathode of the second Zener diode and a collectorterminal connected to both the anode of the second Zener diode and thecathode of the first Zener diode.
 9. The power converter according toclaim 1, wherein the adjustment circuit includes a first switch that isturned off in the first mode and is turned on in the second mode. 10.The power converter according to claim 9, wherein the protection circuitfurther includes a second switch connected in parallel with the secondZener diode and turned on when the first switch is turned on.
 11. Animage processing apparatus, comprising: an image forming unit configuredto form an image on a sheet; and a power converter configured to supplyelectric power to the image forming unit and including: a transformerincluding a first winding and a second winding; a switching circuitconfigured to switch voltages applied to the first primary winding; apower supply circuit connected to the second winding and configured torectify and smooth a current flowing in the second winding and generatea DC voltage; an adjustment circuit configured to adjust the generatedDC voltage according to an operation mode of the power converterincluding a first mode in which the generated DC voltage is adjusted toa first DC voltage and a second mode in which the generated DC voltageis adjusted to a second DC voltage that is less than the first DCvoltage, and output the adjusted DC voltage; and a protection circuitincluding a first Zener diode and a second Zener diode and configured tostop output of the adjusted DC voltage, each of the first and secondZener diodes having a cathode and an anode, the cathode of the firstZener diode being electrically connected to the power supply circuit,the second Zener diode being connected in series to the first Zenerdiode in the first mode and being short-circuited in the second mode,the anode of one of the first and second Zener diodes being directlyconnected to the cathode of the other of the first and second Zenerdiodes via a wire.
 12. The image processing apparatus according to claim11, wherein the protection circuit further includes a light emittingdiode (LED) that emits light when a current flows in the first Zenerdiode, and the switching circuit includes a phototransistor and isconfigured to stop applying the voltages to the first winding when acurrent flows in the phototransistor.
 13. The image processing apparatusaccording to claim 11, further comprising: a photocoupler comprising: alight emitting diode that emits light when a current flows in the firstZener diode, and a phototransistor connected to the switching circuitand coupled to the light emitting diode.
 14. The image processingapparatus according to claim 11, wherein the protection circuit furtherincludes a switch connected in parallel with the second Zener diode andturned on in the second mode.
 15. The image processing apparatusaccording to claim 14, wherein the anode of the first Zener diode isconnected to both the cathode of the second Zener diode and the switch.16. The image processing apparatus according to claim 15, wherein theswitch is a transistor including a collector terminal connected to thecathode of the second Zener diode and an emitter terminal connected tothe anode of the second Zener diode.
 17. The image processing apparatusaccording to claim 14, wherein the switch is connected in series to thefirst Zener diode and the power supply circuit therebetween.
 18. Theimage processing apparatus according to claim 17, wherein the switch isa transistor including an emitter terminal connected to the cathode ofthe second Zener diode and a collector terminal connected to both theanode of the second Zener diode and the cathode of the first Zenerdiode.
 19. The image processing apparatus according to claim 11, whereinthe adjustment circuit includes a first switch that is turned off in thefirst mode and is turned on in the second mode.
 20. The image processingapparatus according to claim 19, wherein the protection circuit furtherincludes a second switch connected in parallel with the second Zenerdiode and turned on when the first switch is turned on.